In general, the light gathering capability of a lens varies between the lens center and the lens periphery, and the lens has a characteristic that the light gathering capability decreases toward the lens periphery. By the influence of this characteristic, when objects having actually the same brightness are photographed, a phenomenon may occur in which a signal level difference occurs in images outputted through an image pickup apparatus. This phenomenon is called a peripheral light amount drop.
A recent image pickup apparatus normally includes a device for correcting the peripheral light amount drop. In such an image pickup apparatus, the peripheral light amount drop is generally corrected by amplifying an image signal, which is outputted through a lens and an image pickup sensor, according to the degree of the peripheral light amount drop.
In the image pickup apparatus as stated above, in a lens peripheral part where the peripheral light amount drop remarkably occurs, an amplification factor (hereinafter referred to as a light amount drop correction gain) required for correction becomes large. Since a random noise is generally included in the image signal, as the peripheral light amount drop correction gain becomes large, the noise is amplified, and the S/N (Signal/Noise) ratio of the image signal is reduced. Thus, although the general peripheral light amount drop correction technique can correct the peripheral light amount drop at the lens peripheral part, there is a problem that the S/N ratio at the lens peripheral part is liable to be reduced.
In order to solve this problem, Patent Literature 1 (JP-A-2004-48562) proposes an image pickup apparatus. The apparatus disclosed in Patent Literature 1 is a peripheral light amount drop correcting apparatus to reduce noise by continuously adjusting a noise reduction effect according to the peripheral light amount drop correction gain.
However, in the foregoing related art, although the peripheral light amount drop at the lens periphery is corrected, and the reduction of the S/N ratio at the lens periphery can be suppressed, deterioration in resolution feeling at the lens periphery is problematic.
In a lens, resolution capability varies between the lens center and the lens periphery, and the resolution capability decreases toward the periphery. Besides, in a noise removing technique using a spatial filter (noise removing unit in the space direction) as typified by a median filter used in the related art, the defect of the deterioration in resolution feeling can not be avoided although the degree of the defect varies according to the algorism excellence.
Thus, in the related art, first, the lens characteristic is such that the resolution feeling at the lens periphery is lower than that at the lens center, and further, the deterioration in resolution feeling as the side effect of the noise reduction process by the spatial filter is added. Accordingly, a difference in resolution feeling is liable to occur between the lens center and the lens periphery.